This invention relates to thermally imageable films, and to a release coating for such films.
Infrared imaging is a form of thermal imaging that involves the use of a focused infrared lamp to heat an infrared absorbing image, commonly referred to as the "original", which image is in contact with a substrate, e.g., a transparent polymeric film, having thermally sensitive imaging chemicals applied to a major surface thereof. Upon image-wise absorbing the focused infrared radiation, the original transfers the absorbed heat to the thermally sensitive imaging chemicals on the surface of the substrate, thereby causing a chemical reaction which results in the formation of a copy of the image of the original on the substrate.
It is frequently desirable to prepare projection transparencies, e.g. transparencies for overhead projectors, from originals which are actually plain paper copies that have previously been prepared from electrophotographic imaging processes. The electrostatic latent image on such a plain paper copy is developed by the application and fixing of toner powder to the plain paper copy. Toner powder is generally a blend of polmer having low melting point, and carbon. When the toner on the surface of a plain paper copy in contact with the substrate from which the projection transparency is to be prepared absorbs infrared radiation, partial remelting of the toner powder on the copy is likely to occur. The portions of the original which bear the remelted toner powder will frequently adhere to the transparency. When the original is separated from the transparency, toner powder from the original is likely to be removed from said original and simultaneously transferred to the surface of the thus-formed projection transparency. This transfer of toner powder reduces the optical density of the image on the original and may, in effect, destroy the quality of the image. Thus, the original can be damaged when a projection transparency is made from it. The adherence of the toner powder to the projection transparency may also result in undesirable effects on the surface of the transparency itself. When the image formed on the surface of the transparency is black, the toner powder does not harm the image itself, but the powder may be rubbed off the transparency and transfer to surfaces which subsequently come in contact with the transparency. When the image formed on the transparency is a color, the toner powder can cause the colored image to have irregular black spots in the colored image area. This is considered to be a major defect in the transparency. A barrier film interposed between the imageable layer of the transparency and the original can prevent toner powder from being picked up and retained by the transparency. In a type of color transparency currently in use, a film containing an acid does serve as such a barrier.
In addition to the foregoing problems, certain imageable materials tend to liberate moisture upon exposure to heat or infrared radiation. This moisture liberation results in formation of opaque areas, i.e. "halos", around the edges of the images. These areas scatter light and project as darkness around the image.
Ito, et al, U.S. Pat. No. 3,955,035 discloses a trialkoxy silane coating which imparts abrasion resistance, hardness, and release properties to plastics. This coating, however, is brittle and will crack if applied to a flexible polyester substrate of the type commonly used for preparing transparencies. Clark, U.S. Pat. No. 3,986,997 discloses a coating formed from a dispersion of colloidal silica in a condensate of methyl trihydroxy silane. This coating is also brittle, and, thus, it is unsuitable for flexible sheeting. Baney, et al, U.S. Pat. No. 4,223,072 discloses a coating formed of phenyl trihydroxy silane. Although this coating exhibits flexibility superior to that of the coating disclosed in the Clark patent, the flexibility is insufficient to allow coating on thin polyester films. Grenoble, U.S. Pat. No. 4,071,644 discloses a flexible sheet material coated with siloxanes which is useful as a non-adherent surface. The coating composition in this patent comprises vinyl alkyl siloxane oligomers, alkyl hydrogen siloxanes, and a catalyst. These coatings are curable at 250.degree. F. (121.degree. C.), a temperature at which a temperature sensitive coating such as that required for infrared imageable films and thermally imageable films would react prematurely. Garden, et al, U.S. Pat. No. 3,936,581 discloses a release coating containing vinyl siloxanes in mixture with alkyl hydrogen siloxanes and a platinum catalyst. The optimum cure temperatures are in excess of 100.degree. C., a temperature which would bring about premature reaction of the temperature sensitive coatings of infrared imageable films.